Gas turbines are used in many fields for driving generators or driven machines. In this case, the energy content of a fuel is used for creating a rotational movement of a turbine shaft. For this purpose, the fuel is combusted in a combustion chamber, wherein compressed air is supplied by an air compressor. The operating medium, at high pressure and at high temperature, which is produced in the combustion chamber as a result of the combustion of the fuel, is guided through a turbine unit, which is connected downstream to the combustion chambers, where the operating medium expands, performing work.
For creating the rotational movement of the turbine shaft, a number of rotor blades, which are customarily assembled into blade groups or blade rows, are arranged on this turbine shaft and via an impulse transfer from the flow medium drive the turbine shaft. For guiding the flow medium in the turbine unit, moreover, stator blade rows which are connected to the turbine casing are customarily arranged between adjacent rotor blade rows. For suitable guiding of the operating medium, the turbine blades, especially the stator blades, customarily have a profiled blade airfoil extended along a blade axis and upon which a platform, which extends transversely to the blade axis, is formed onto the end face for fastening the turbine blade on the respective carrier body. The upper side of the platform which faces the blade airfoil forms an outer delimiting surface for the flow passage of the gas turbine which guides the hot gas.
For the simple and secure installing and fixing on a stator blade carrier which is connected to the turbine casing, the platform of the respective stator blade customarily has a number of hook-like fastening elements on its underside or rear side which faces away from the blade airfoil. A similar type of hook fastening of a guide ring which bridges the axial gap of two stator blades of adjacent turbine stages is known for example from EP 1 505 259 A1. For the installing, the stator blade with its fastening elements or fastening hooks is inserted, aligned and then fixed in a suitable manner, for example by means of caulking plates, in corresponding locating slots of a stator blade carrier. In the case of the aforementioned guide ring, according to EP 1 505 259 A1 an additional fastener is also provided, by means of which the hook of the guide ring can be further clamped in the stator blade carrier.
Moreover, the fastening of a stator blade of the first turbine stage is known from U.S. Pat. No. 2,942,844. The stator blade comprises an inner platform upon which a flange, which extends transversely to it, is welded. For fastening the stator blade, the flange, which is provided with a hole, is fastened in a non-positive-locking manner on a support structure by means of a screw which extends through the hole.
For reducing the production or installation cost, a plurality of stator blade airfoils of a stator blade row, which are adjacent to each other in the circumferential direction of the gas turbine, can also be arranged on a common platform, so that the complete blade unit, which is subsequently referred to as a stator blade segment, can be inserted axially or in the circumferential direction into the associated stator blade carrier by means of the fastening hooks on the platform side. For simplification of the way of speaking, the term “stator blade segment” in the following text, especially also in the claims, is always to also include the case of an individual stator blade with only one blade airfoil, provided that this is not specifically excluded.
The stator blade or the complete stator blade segment is customarily produced within the scope of a casting process so that the platform and the fastening elements on the platform side are integral co-cast component parts of the stator blade or of the stator blade segment. For this purpose, in a first step a so-called wax model of the blade or of the blade segment is manufactured and then provided with a ceramic coating as a result of repeated immersing in a ceramic mass. As soon as this has a sufficient thickness, the wax model which is provided with the ceramic coating is burned out, wherein the ceramic hardens and the liquefied or evaporated wax is removed. The negative casting mold of ceramic which is obtained in this way is finally cast with the metal blade material. After solidification of the melt and the removal of the shell-like outer casting mold, ceramic core elements which possibly still remain in the blade body and which were previously introduced for the forming of cavities or cooling passages which are integrated in the blade bodies are removed by leaching with caustic soda or the like.
The fastening elements, which project like a hook from the platform, create difficulties within the scope of the manufacturing process in several aspects. The production of the wax model is already relatively complicated since for forming the fastening hooks comparatively complex wax molds with a large number of so-called masking elements or slides are required. Also, the fastening hooks represent problem areas with regard to casting technique, since the undercuts when constructing the mold shells can only be poorly sanded and during the subsequent casting process, on account of their exposed position, are always prone to the formation of blowhole fields, i.e. to material defects which are created as a result of heat shrinkage in the component as it cools down.
Moreover, it is frequently difficult to meet the tolerances which are required for an accurately fitting seating of the fastening hook in the associated locating slot, especially in the case of embodiment variants with comparatively small radii of curvature. As a result, sealing problems can also occur at this point during subsequent operation of the turbine. Finally, it has been shown that the fastening hooks often also represent weak points of the turbine blades with regard to their wear characteristic under operational load and with regard to the permissible maximum load input.